Does a standby generator pay vs a home battery for backup?

How this estimates

Outage events a year are outage hours divided by hours in one outage. The generator covers every outage hour if the power you want is within its rating, and none if it is above it.

The battery keeps the share you choose for outages. It runs your load for that energy divided by the load, in hours. In each outage it covers the shorter of the outage length and that run time, and it covers nothing if the load is above the power it can supply at once.

Generator gas use is a straight line through the two published points, half load and full load. That straight line is an assumption. The weekly test run is priced at the gas use with no load, which is on the high side because the unit runs slowly. Gas cost is cubic feet times the price.

The battery saves on your bill only with the part it does not keep for outages. Solar moves in first, and each kilowatt-hour saves the grid price times the efficiency, less the export credit it gave up. On a time-of-use plan, each evening discharge saves the energy used times the peak price, less the energy needed to charge it times the off-peak price. A flat rate with no solar saves nothing.

Each option goes through the same cash-flow math: the price after any rebate, running costs and savings that rise by the yearly price rise, and the same price again in the year after a unit wears out. There is no resale value and no discount rate. Cost per outage hour covered is the net cost divided by the years times the hours covered a year. The break-even value of an outage hour is the net cost divided by all the covered hours, with the yearly rise, and it is the value at which cover alone would repay the cost.

The flat-price stress test runs the same math with the yearly rise set to 0. The two biggest changes come from moving outage hours, power to keep on, both prices, the gas price, the cost of an outage hour and (on a time-of-use plan) the peak gap by 25% each way, and comparing the net cost of each option after counting the value of cover.

Left out: resale value, battery fade (enter the end-of-life size to see the worst case), recharging a battery from solar during a long outage, a mix of outage lengths (every outage is the average length), the chance that a generator fails to start or a battery is low when the grid drops, home value, insurance, noise, fuel delivery, permits and local rules. Propane, diesel and portable units are not modeled; edit the gas boxes to try propane.